Objectives: The major objectives are (1) to acquire data which will permit a more thorough understanding of the means used by trematode parasites to avoid immunologic attack by intermediate hosts, and (2) to seek to understand cellular mechanisms of immunologic response in molluscan schistosomiasis. The work should lead to a fuller understanding of host-evasion and host-interference techniques used by parasites, and may reveal key factors permitting the recognition of foreign identity. The objectives are pertinent both immediately, to schistosomiasis, and more basically to fundamental problems in cell biology. Methodology: Biological materials will be strains of Biomphalaria glabrata and strains of Schistosoma mansoni, selected for differences in compatibility. Susceptible snails of different ages and either infected or uninfected will also be used. Serological and biochemical approaches will be used to "profile" plasma from these various snails and to examine plasma membrane components of snail cells and of sporocyst teguments. Both in vivo and in vitro preparations will be used to evaluate snail cell-sporocyst interactions at the ultrastructural level. Emphasis will be on mechanisms of sporocyst recognition by snail cells, sporocyst evasion of host attack, and sporocyst damage by snail cells.